Electro-optic display

ABSTRACT

An electro-optic display includes a “matrix” for confining moving elements of the display (e.g., rotating or twisting elements). The matrix (or at least the viewable portions thereof) may have a high reflectivity, comparable to that of white paper. This results in an overall “whiter” or brighter display. The matrix may include channels to facilitate inter-cell fluid transport and high-density element packing. In some cases, the matrix elements provide a hexagonal arrangement of cells for holding the rotating elements. The rotating elements of the display may be electrically and optically anisotropic hemispherically coated spheres. The hemispherical coating typically provides the necessary charge to create electrical anisotropy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC 119(e) from U.S.Provisional Patent Application No. 60/850,883 (Attorney Docket No.DSI1P003P) naming Lipovetskaya et al. as inventors, titled“Electro-Optic display” filed Oct. 10, 2006, which is incorporated byreference in its entirety and for all purposes.

FIELD OF THE INVENTION

The present invention relates to visual displays. Specifically, itrelates to electro-optic displays. More specifically, it relates tofront plane designs for electro-optic displays.

BACKGROUND OF THE INVENTION

Visual displays that make use of ambient light to illuminate theirpixels (reflective) and that produce an image that is indefinitelystable in the absence of electrical input are often referred to aselectronic paper, since they mimic some of the most advantageousproperties of paper. Just like white paper that reflects and scattersincident light and does not require additional light sources for viewingthe images printed upon it, electronic paper displays reflect andscatter ambient light in the white or light colored areas (oftenimage-free areas) and absorb light in the black or dark color areas(often where the image appears). Thus, an electronic paper display canprovide images that are viewable in the absence of backlight or pixelemission illumination (e.g., light emitting diode pixels). The absenceof backlight makes such displays more pleasing to the eye, since theappearance of an image on such display resembles the appearance of animage on a sheet of paper. Further, since a backlight source is notrequired for these displays, they can be manufactured in less bulky,thin forms that may also possess some paper-like flexibility.

Electronic paper displays may also be bistable. Bistability refers tothe ability of an image to remain stable in the absence of externalstimuli (e.g., an applied electric potential). In bistable displays thestates of individual pixels (e.g., whether the pixels are light or dark)remain intact for long periods of time when no external potential isapplied to the display. Therefore, images can be stored on bistabledisplays for a prolonged time without the need for continuousapplication of power, much like images stored on paper. This makesbistable displays especially appealing for portable-displayapplications. Further, since power is consumed by bistable displays onlywhen the image is changed, these displays are more economical for someapplications than conventional LCD and CRT displays. In CRT displays,for instance, the image needs to be constantly refreshed. While lowrefresh rates can conserve some power, this often results in flickeringof the display and consequent eye strain of the viewer.

The image on the electronic paper display can be changed when desired,allowing a variety of applications for such displays. In one example,such displays serve as “reusable paper” for displaying still images. Inother examples, they are used to display real-time moving imagery invideo applications.

The first electronic displays with paper-like properties were developedin the 1970s at Xerox's Palo Alto Research Center. These displays, oftenreferred to as “gyricon” displays, are based on rotation of opticallyand electrically anisotropic spheres embedded in an elastomer. In oneexample of a gyricon display, each sphere is composed of negativelycharged black wax or plastic on one side and positively charged whitewax or plastic on the other side. Each sphere is suspended in adielectric fluid contained within a cavity formed in a plasticizedelastomer. Each sphere is free to rotate in the fluid so that it couldturn with black or white side to the viewer, thus providing a pixel witha black or white appearance. When an appropriate voltage is applied tothe electrodes addressing selected spheres, the spheres rotate inaccordance with their dipole moment and display an image to the viewer.

Gyricon technology, however, failed to produce image quality comparableto that of images printed on paper. In particular, gyricon displays didnot possess the high reflectance of white paper, therefore providinglow-contrast images. Gyricon displays also had limited environmentalstability, because plasticized polymer was not capable of withstandinghigh-temperature or high-humidity conditions. Further, only fewdielectric fluids were suitable for use in gyricon displays, sincedielectric fluid in gyricon was serving both as a polymer plasticizerand as a rotation media and therefore had to possess properties suitablefor both of these applications.

The brightness and contrast of displayed images is primarily determinedby the maximum reflectance that a display may attain. The overallreflectance of the display is influenced by the quality of optically andelectrically anisotropic spheres as well as by optical properties of thematerial filling the gaps between individual spheres. Although inimproved versions of gyricon, described in U.S. Pat. No. 5,754,332issued to Crowley et al., these gaps are minimized by employing aclosely packed monolayer of bichromal spheres, this improvement wasstill very far from sufficient to approach the paper-like reflectance ofabout 85%. Even in a closely packed monolayer there remains someelastomer or matrix material occupying gaps between the spheres, whichreduces the observed reflectance of the display and, hence, the contrastand brightness of displayed images. Since gyricon technology largelyrelies on swelling of elastomer to encapsulate the rotating spheres, theportions of elastomer filling the interstitial regions between thespheres, typically enlarge upon swelling, and absorb a significantamount of light, even when a closely packed monolayer of spheres isemployed.

U.S. Pat. No. 5,815,306 issued to Sheridon et al. describes an improvedgyricon display having an “eggcrate” matrix for holding individualspheres. The matrix provides a geometrically ordered array of cavitiesfor containing the spheres, with one sphere residing in each cavity. Thematrix is used in order to align auxiliary optical devices with thespheres, so that the display can function in a light transmission mode,transmitting or obscuring the passage of light so as to create an image.Such a matrix, although useful as a holding and aligning element, doesnot address the problem of low reflectance in the areas between thespheres, and, consequently, does not improve the contrast and brightnessof the display.

Therefore, there is a need for an electronic paper display that canprovide high-contrast images. Preferably, such display will have anoverall reflectance that is comparable to reflectance of paper. Itshould be suitable for viewing both still and moving imagery, and shouldallow fabrication in thin and flexible forms. In addition, such displayshould preferably be robust and environmentally stable, e.g., it shouldbe capable to withstand high-temperature and high-humidity conditions.

SUMMARY

The present invention provides various improvements over knownelectronic paper, and particularly over known gyricon displays. Some ofthese improvements reside in the use of an inventive “matrix” forconfining individual rotating or twisting elements of the display. Forexample, the matrix (or at least the viewable portions thereof) may havea high reflectivity, comparable to that of white paper. This results inan overall “whiter” or brighter display. Other examples of improvementsinclude matrix designs with channels to facilitate inter-cell fluidtransport and high-density element packing. Some of these improvementsderive from improved fabrication techniques which will be describedherein. Further, other improvements reside in the use of coated rotatingelements (e.g., hemispherically coated opaque spheres). The coatingtypically imparts an electrical charge to the elements, and thus createselectrical anisotropy.

In one aspect, the invention provides a front plane for an electro-opticdisplay. In one embodiment, the front plane includes a first sideadapted for electrical communication with a backplane and at least oneelectrode on a second side of the front plane opposite to the firstside. Further, the front plane includes a matrix having a plurality ofcells facing a viewable surface of the front plane and interstitialregions outside the cells also facing the viewable surface of the frontplane, where the interstitial regions have a first color. The frontplane further includes a plurality of optical elements disposed in theplurality of the cells of the matrix. The optical elements have thefirst color for display when a pixel of the electro-optic display is ina first electrical state.

In one embodiment, a front plane for an electro-optic display includes afirst side adapted for electrical communication with a backplane and atleast one electrode on a second side of the front plane opposite saidfirst side. The front plane further includes a matrix having a pluralityof cells defined by walls in the matrix. Further, a plurality ofoptically and electrically anisotropic elements are disposed in theplurality of matrix cells. The optically anisotropic elements have atleast first opaque exterior region of a first color and a second opaqueexterior region of a second color. It is understood that the interiorportions of the elements can have the same or a different color (andmaterial) from the exterior portions. In different embodiments solid orhollow optically anisotropic elements may be used. The front planefurther includes a fluid provided in the matrix cells, such that theoptically anisotropic elements can rotate from a first orientationdisplaying the first color to a second orientation displaying a secondcolor when an electric field is applied to their cells.

In yet another embodiment, a front plane comprises a light coloredmatrix having a plurality of cells defined by walls in the matrix. Thefront plane also comprises a plurality of optically anisotropic elementsdisposed in the plurality of cells, where the optically anisotropicelements have at least two colors. When these elements are in the firstorientation the light color of the elements is presented to the firstside of the front plane and when these elements are in the secondorientation a darker color is presented to the first side of frontplane.

In another aspect, the invention provides a rotating element display. Inone embodiment, the display includes a back plane containing a pluralityof electrodes distributed in two dimensions on the back plane, whereineach of these electrodes allows independent control of a discrete regionof the display. In some embodiments the back plane electrodes may besubstantially coplanar. The display also includes a front plane havingtwo sides, wherein one side is connected to or is proximate the backplane. At least one electrode is located on another side of the frontplane opposite the back plane. The front plane includes a matrix whichprovides a plurality of cells defined by walls in the matrix and whichhas at least one channel through at least some of the walls connectingat least some of the cells with one another. The front plane alsoincludes a plurality of optically and electrically anisotropic elementsdisposed in the cells of the matrix. The cells are filled with fluid sothat the elements can twist or rotate from a first orientation to asecond orientation within their respective cells when an electric fieldis applied to the cells.

In one embodiment the optically anisotropic elements have at least twocolors. Typically, when these elements are in the first orientation onecolor is presented to the first side of the front plane and when saidelements are in the second orientation a different color is presented tothe first side of the front plane. Preferably, one color of the at leasttwo colors is lighter than a second color of the at least two colors.For example, the at least two colors may be black and white. In oneembodiment the rotating elements are spheres that may have an averagediameter of about 25-150 micrometers.

In some embodiments, the matrix has multiple channels connectingmultiple cells along the path. For example, these channels can besubstantially parallel to one another and can be connecting cells inrows. In other embodiments the channels may be intersecting, e.g., theycan be substantially perpendicular to each other. Preferably, thesechannels are arranged so as to allow the dielectric fluid to be drawninto the front plane during its assembly. In one example, at least someof the channels have a cross sectional area of at least about 5% of thecross sectional area of the cell. In some embodiments the walls of thecells are comprised of “posts”, and the fluid is allowed to flow freelyin the channels between the posts. In some embodiments, the matrix isdesigned such that at least one cell in the matrix is in fluidcommunication with each of its adjacent cells. In some embodiments thematrix includes a plurality of regions, wherein the cells within thesame region are in fluid communication with one another, and the cellsfrom different regions are separated from one another by a wallpreventing fluid communication between adjacent regions of the matrix.For example, a wall can separate regions every 2-100 (e.g., every 5-50)rows of cells. Such matrix design minimizes the chances for failure ofthe display.

In some embodiments the matrix comprises walls having different heights.For example, the matrix can include supporting walls and arrangingwalls, wherein the supporting walls have a height that is at least equalto a height or a diameter of an optically anisotropic element, andwherein the arranging walls have a height of less than about 80% (e.g.less than about 50%) of the height (or diameter) of an opticallyanisotropic element.

In some embodiments, the matrix is designed such that the height of thecells allows for a translational movement of an optically anisotropicelement in a cell in a direction defined by the first and second sidesof the front plane (typically to the viewer and away from the viewer).For example, the height of the cell may be at least 1.1 (e.g., 1.5)times greater than the height or a diameter of an optically anisotropicelement residing in a cell.

The matrix can host one optically anisotropic element per cell, or canprovide cells large enough to host a plurality of elements. In someembodiments, the matrix can host a plurality of elements which can bedisposed as a monolayer. In other embodiments, several layers ofoptically anisotropic elements may reside in one cell. For example,between about 2-5, preferably about 2-3 layers can reside in the cell.

In order to maximize reflectance of the display, the cells of the matrixcan be arranged in a hexagonal close packed pattern, so as to minimizeinterstitial area between the rotating elements disposed in the matrixcells. The interstitial area can be further minimized when the matrixwalls separating adjacent cells are thin. For example, the walls thatdefine cells of the matrix may have a minimum width at regionsseparating adjacent cells of at most about 45 micrometers, and at most 5micrometers in certain embodiments. The appropriate wall thicknesses maycorrespond to high aspect ratios of the matrix walls. For example,matrix walls at positions of their greatest height and least width, canhave an average aspect ratio of at least about 5:1 (height to width),preferably at least about 8:1. It is preferable, that an area projectedby the cells on the first side of the front plane occupies at least 65%of a corresponding area of the first side. It is also preferable, thatat least a portion of the matrix visible through the first side of thefront plane has a light color. For example, the front plane of thedisplay may have a diffuse reflectance of at least about 30%.

In other embodiments, it is advantageous to use a square close packdesign in a matrix. The square pack design can achieve a good fillfactor and can also make use of the walls with a lower aspect ratio.Therefore such design can be more easily manufactured, and can bestructurally more stable than a hexagonal design having high aspectratio walls. In some embodiments, the walls (e.g., posts) of a matrix ina square pack design have an aspect ratio of less than about 5:1, e.g.,about 4:1.

In some embodiments the advantageous properties of hexagonal and squarepack designs are combined, by providing a matrix having a first regionin which the cells are arranged in a square close packed pattern and aregion in which the cells are arranged in a hexagonal close packedpattern.

In another aspect, the invention provides a method of assembling a frontplane of a twisting element or rotating element display. A matrix havinga support surface, a plurality of cells defined by walls on the supportsurface in the matrix and at least one channel through at least some ofthe walls and connecting at least some of the cells with one another, isprovided. A plurality of optically anisotropic elements is disposed inthe plurality of cells of the matrix. At least one electrode on a sideof the matrix opposite the support surface is provided. A dielectricfluid is drawn into the cells of the matrix through the at least onechannel. The front plane produced by this method allows the elements totwist from a first orientation to a second orientation within theirrespective cells when an electric field is applied to the cells. At anappropriate point in the fabrication process, the front plane may beattached to the back plane. The front plane may be assembled by coatingone side of the front electrode layer with an adhesive (e.g., with anoptically transparent heat-activated adhesive); contacting the coatedelectrode side with the matrix containing the optically anisotropicelements; and attaching the electrode to the matrix at the sidecomprising cell openings with the adhesive. When heat-activated adhesiveis used, the attachment of the matrix to the front electrode isaccomplished by heating the assembly comprising the matrix and theelectrode.

In another aspect, the invention provides a method for loading elementsinto cells of a matrix of a partially-fabricated electro-optic display.In one embodiment the method involves forming a suspension of theelements in a fluid, wherein the fluid is inert towards an exteriormaterial of the elements; contacting the suspension with an absorbenttransfer member (e.g., a brush) to load the transfer member with aplurality of elements; contacting the matrix with the loaded transfermember to transfer the elements into the cells of the matrix; andremoving excess elements from the surface of the matrix (e.g., with asecond brush). The method can be applied to load very small elements(e.g., elements with a diameter less than about 1 mm, less than 0.15 mm,e.g., about 50 μm) into small matrix cells. For example, the elementscan be transferred such that one optically anisotropic element occupieseach matrix cell.

In another aspect, the invention provides a method of using the rotatingelement display. The rotating element display is suitable for viewingboth still and moving images. The images are created by providing aplurality of signals to at least some of the electrodes of the display,so that a potential difference between electrodes is created in responseto these signals. The optically and electrically anisotropic elementsare rotated in response to said potential difference from a firstorientation to a second orientation. In one embodiment, one color ispresented to the first side of the front plane when the element is infirst orientation and a different color is presented to the first sideof the front plane when the element is in second orientation. Thesignals provided to electrodes can selectively address specificelectrodes, wherein each electrode allows independent control of adiscrete region of the display. Therefore, an image can be created inresponse to these signals. In some embodiments, the image thus createdwill have high contrast due to high reflectance of the inventivedisplay.

These and other features and advantages of the invention will bedescribed in more detail below with reference to the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional side view of a twisting elementdisplay in accordance with one embodiment of the present invention.

FIG. 2 shows a top view of a twisting element display illustratinghexagonally close-packed monolayer of spheres in accordance with oneembodiment of the present invention.

FIG. 3A shows a side view of a bichromal sphere suitable for a twistingelement display.

FIG. 3B shows a cross-sectional view of the sphere presented in FIG. 3A.

FIG. 4 presents selected examples of charge distributions suitable forproviding electrical anisotropy to the spheres.

FIG. 5A shows an isometric view of a matrix structure illustrating ahexagonal close pack of matrix cells with parallel channels inaccordance with one embodiment of the invention.

FIG. 5B shows a top view of the matrix structure presented in FIG. 5A.

FIG. 6A shows an isometric view of a matrix structure illustrating ahexagonal close pack of matrix cells with parallel channels inaccordance with another embodiment of the invention.

FIG. 6B shows a top view of the matrix structure presented in FIG. 6A.

FIG. 6C shows a cross-sectional side view of an individual cell of thematrix presented in FIG. 6A.

FIG. 7A shows an isometric view of a matrix structure illustrating asquare close pack of matrix cells with parallel and perpendicularchannels in accordance with an embodiment of the invention.

FIG. 7B shows a top view of the matrix structure presented in FIG. 7A.

FIG. 7C shows a cross-sectional side view of the matrix presented inFIG. 7A.

FIG. 8A shows an isometric view of a matrix structure illustrating asquare close pack of matrix cells with parallel and perpendicularchannels in accordance with an embodiment of the invention.

FIG. 8B shows a top view of the matrix structure presented in FIG. 8A.

FIG. 8C shows a cross-sectional view of the matrix presented in FIG. 8A.

FIG. 9A shows an isometric view of a matrix structure illustrating ahexagonal close pack of matrix cells without a channel in accordancewith an embodiment of the invention.

FIG. 9B shows a top view of the matrix structure presented in FIG. 9A.

FIG. 9C shows a cross-sectional side view of individual cells of thematrix presented in FIG. 9A.

FIG. 10A shows an isometric view of a matrix structure illustrating asquare close pack of matrix cells with parallel and perpendicularchannels and with cell walls having different heights in accordance withan embodiment of the invention.

FIG. 10B shows a top view of the matrix structure presented in FIG. 10A.

FIG. 10C shows a cross-sectional side view of the matrix presented inFIG. 10A.

FIG. 11A shows an isometric view of a matrix structure illustrating acombination of a square close pack and a hexagonal close pack of matrixcells in one matrix in accordance with an embodiment of the invention.

FIG. 11B shows a top view of the matrix structure presented in FIG. 11A.

FIG. 12A shows an isometric view of a matrix structure illustrating ahexagonal close pack of matrix cells, in which each cell is designed toaccommodate multiple elements, in accordance with an embodiment of theinvention.

FIG. 12B shows a top view of the matrix structure presented in FIG. 12A.

FIG. 12C shows a detailed view of the matrix presented in FIG. 12A.

FIG. 12D shows a cross-sectional view of the matrix presented in FIG.12A.

FIG. 13A presents a possible process flow diagram for a method ofassembly of a twisting element display.

FIG. 13B presents another possible process flow diagram for a method ofassembly of a twisting element display.

FIG. 14 presents a process flow diagram for a method of loading theoptically anisotropic elements into the matrix of an electro-opticdisplay in accordance with one embodiment.

FIGS. 5-12 provide dimensions for different matrix elements as anexample. The dimensions are shown in rectangular blocks and refer todimensions in micrometers, except for FIG. 11B, where the dimensions areshown in millimeters. It is understood that drawings are used forillustration purposes, and that the invention is not limited toparticular dimensions shown.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Introduction and Overview

Electro-optic displays of present invention may possess a number ofadvantageous characteristics that may compare favorably to gyriconelectronic paper. These displays may provide high brightness andcontrast images (due, for example, to specific properties of thematrix). They also may possess high resolution due to, for example,small pixel size. Further, the displays of present invention may bemanufactured from environmentally robust materials, resulting inenvironmentally stable displays (e.g., displays withstanding hightemperature and high humidity conditions). The design of these displaysallows for easy display manufacturing, making use, in certainembodiments of a channel in the matrix during display assembly. Displayassembly methods are also provided in some aspects of present invention.

The simplicity of electro-optic displays provided by this inventionminimizes the number of components, and thus cost of displaymanufacturing. In some embodiments, the displays of present invention donot require backlight, and thus reduce the power consumption duringdisplay operation.

As explained, electrically anisotropic elements having a non-uniformcharge distribution on their surfaces can be caused to move in discreteelectric fields. This movement can be coupled with a change in theobserved optical properties of the element, so that the viewer willobserve different appearance of the element (e.g. black or white)depending on the direction and the strength of the field. Appropriatelyordered and contained addressable arrays of such elements can be used aselectronic paper displays since they do not require backlight and can befabricated in thin and flexible forms. They are also often bistable,thus providing images that are stable in the absence of externalelectric input.

Since these displays rely on response of elements to an electricstimulus to produce an optical effect, they are sometimes referred to aselectro-optic displays. Electro-optic displays in which elements rotateto produce optical effects are sometimes referred to as twisting elementor rotating element displays. While element movement can be bothtranslational and rotational, it is sometimes advantageous to constrainthe movement to rotational movement only (or rotational movement coupledwith minimal translational movement). As explained more fully below,this is because translation requires a space for the elements totranslate, which generally means that the electrodes driving movement ofthe elements must have a greater separation distance (in comparison tothe case where the elements are constrained to rotation only). Thisgreater separation requires a corresponding greater driving voltagebetween the electrodes to effect rotation. Displays limited torotational movement generally require smaller driving voltages andresult in faster response times.

Therefore, in certain embodiments, twisting element displays of thisinvention employ only rotational movement of electrically anisotropicbichromal spheres. It should be noted, however, that in some embodimentsof the present invention, there may exist a translational component tothe movement of the elements, in conjunction with the rotationalcomponent. Rotating elements of this invention are typically sphericalin shape. This allows the use of simple matrix designs for housing therotating elements. For example, the matrix may comprise an array ofgenerally cylindrical cells to house the rotating elements. It should beunderstood, however, that in certain embodiments non-spherical rotatabledisplay elements, such as cylinders, may be used, and a matrix withrectangularly-shaped cells may be appropriate.

Referring to FIG. 1, a cross-sectional view of a display structure inaccordance with one embodiment of the present invention is illustrated.The viewable direction is indicated by an arrow 101. Electricallyanisotropic bichromal spheres 103 are rotatably disposed within cells105 defined by matrix walls 107, a front electrode layer 109, and amatrix base 111. A backplane 113 is attached to the matrix base 111. Thespheres are immersed in fluid 115, so that they can freely rotate whenan electric field is applied. A plurality of electrodes 117 (no detailsdepicted) are distributed in two dimensions on the backplane 113, sothat each electrode can independently control a discrete region of thedisplay, typically a single cell and sphere. Alternatively, oneelectrode can control multiple spheres. In one example, each sphere andassociated backplane electrode together corresponds to one pixel on thedisplay.

The front electrode layer 109 typically contains one or more electrodesand is usually composed of a conductive transparent material, such asindium/tin oxide (ITO) coated on polyethyleneterephthalate (PET). Otherconductive transparent materials suitable for front electrode layerinclude conductive polymers (e.g. PEDOT(poly(3,4-ethylenedioxythiophene)), or PSS:PEDOT(poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate)),carbon nanotubes, doped oxide materials, such as aluminum/zinc oxide,and the like. These materials can be used either alone or as coatings ontransparent substrates, such as PET. The front electrode layer should,preferably, have very high light transmissivity. For example,transmissivity of greater than about 82%, preferably greater than about85% is preferred. ITO-PET films with these transmissivity properties arecommercially available from a number of suppliers, such as CPFilms Inc.of Fieldale, Va. and Sheldahl Inc. of Northfield, Minn. Typically, thefront electrode layer is a single sheet of electrode material coveringall or a significant fraction of the pixels (and rotating elements) inthe display. This is in distinction from the backplane electrodes, whereeach electrode is associated with a single pixel. The potentialdifferential between the front and back electrodes creates the necessaryelectric field for rotation of the spheres. In some embodiments, howeverthe front electrode layer may include a plurality of electrodes, whereineach electrode may address individual pixels or individual rotatingelements of the display. Note that the assembly 119 is usually referredto as the “front plane” of the device and includes the front electrodelayer 109, the matrix, the spheres, and the fluid.

In one example, the bichromal spheres 103 have positively charged blackhemispherical coating, while the remaining hemisphere is white anduncharged (or negatively charged). When an appropriate potentialdifference is applied between the electrodes on the back and frontplanes, the sphere in a pixel element will rotate so as to align itscharges with the applied electric field, thereby presenting a black orwhite hemisphere to the viewer. The rotation between states ispreferably by about 180±15°, so that a black or a white hemisphere isfully visible.

In selected embodiments, spheres having more than two opticallydifferent segments or spheres wherein optical properties alternate fromsegment to segment, may be used. For example, spheres havingdifferent-color quadrants or thirds may be used. For these spheres, ofcourse, the degree of rotation should correspond to the optical patternof the sphere. For example a sphere having different-color quadrants maybe rotated to about 90, 180, or 270° depending on the color that needsto be presented to the viewer.

Incident light falls onto the display from the direction shown by arrow101. After it passes through the transparent front electrode layer, itis absorbed or reflected by the spheres, depending on the hemispherethat is presented to the viewer. Light is absorbed on the array of blackhemispheres and is reflected by the white hemispheres, thereby creatinga black image on a white background. If desired, it is possible toproduce a gray image, on a white background. A gray image can beproduced, for example, if selected spheres are not completely rotated.In this case the viewer will see parts of both black and white surfacesof individual spheres, so that an impression of gray color will becreated. Incomplete rotation of the spheres can be achieved by applyingsmaller voltages than those needed for 180° rotation or by applyingvoltage in pulses of short duration, e.g., by pulse width modulation(PWM) methods Other methods of creating gray images can be used as willbe recognized by those skilled in the art. These methods are applicablenot only for creating gray images, when black and white hemisphericallycoated spheres are used, but they can be also employed to rotate amulti-colored sphere to a specific degree that is different from 180°.For example spheres having quadrants or thirds of different color can berotated by 90 or 120° by using pulse width modulation methods.

Light falling into the interstices between the spheres (or otherrotational element) may be reflected, absorbed, or partially absorbed(depending on the optical properties of the displayed interstitialportions of the matrix). The optical properties of the displayed regionoccupied by the matrix are generally not subject to variable control.With this in mind, aspects of the invention, which will be describesshortly, pertain to careful choice of the optical properties of thematrix viewable in the interstitial regions.

Unfortunately, it is not geometrically possible to cover 100% of thedisplay area with the spheres. It is, however, advantageous to minimizethe interstitial area by using a monolayer of close-packed spheres. Forexample, a hexagonal close-pack, illustrated in FIG. 2 presenting a topview of the display, can afford up to π/(2·3^(1/2)) or about 91% arealcoverage by the spheres. Other packs, such as rectangular and rhomboidalarrays afford approximately 78% of areal coverage. All of the mentionedpacks, as well as other arrangements of spheres on a plane known tothose skilled in the art, can be used in the embodiments of presentinvention. Selected packs have been discussed in detail in U.S. Pat. No.5,754,332 issued to Crowley et al., which is herein incorporated byreference in its entirety.

In one embodiment of the invention, the spheres are contained in anordered array (e.g., square or hexagonal), in a containment matrix asclose to each other as possible. The containment cells typically containone sphere per cell, with the cell width being only slightly greaterthan the diameter of the sphere, so as to allow rotation of the sphere,but to minimize its translation. In order to minimize interstitial area,the walls of the containment matrix should be very thin, typically nothicker than about 20 percent of the sphere diameter (at the pointswhere the spheres are closest together). In other embodiments, the wallthickness is no thicker than about 10 percent of the sphere diameter. Insome embodiments, walls that define cells of the matrix may have aminimum width at regions separating adjacent cells of at most about 20%and in some cases at most about 35% of the size (diameter or width) ofthe cells. Preferably, walls with a thickness of at most about 13% or atmost about 10% of the diameter or width of cells, are used. In thoseembodiments when walls are posts, thicker posts with widths of up to 35%of the size of the walls can be employed. In certain embodiments, thewall thickness is at most about 45 micrometers, more preferably at mostabout 10 micrometers, and even more preferably at most about 5micrometers (each value measured at the points of minimal separationbetween the spheres). These close packing constraints may impose variousstructural requirements on the matrix. For example, they may requirethat the matrix walls have an average aspect ratio at positions of theirgreatest height and least width of at least about 5:1 (height to width),preferably at least about 8:1, for example about 10:1. In accordancewith embodiments of this invention, an area projected by the cells ontothe front plane can be at least about 65%, preferably at least about 75%or at least 85% of the total front plane viewable area. These values areonly slightly smaller than the theoretically possible values forrhomboidal and hexagonal packs.

While it is advantageous to minimize the interstitial area between theadjacent rotating elements, as achieved by a matrix with a hexagonalclose pack arrangement of the cells, it is often difficult tomanufacture matrices with very high aspect ratio walls (as oftenrequired for hexagonal close packs). Further, high aspect ratio wallscan be unstable and can be more easily broken during display assembly orduring the end use of the display compared to lower aspect ratio walls.In some embodiments, lower aspect ratio walls, e.g., walls with anaspect ratio of 5:1 and lower are used. In some of these embodiments,the interstitial area is minimized by reducing or eliminating some ofthe wall structure between the spheres. In some embodiments, the cellsare defined by “posts”, arranged to provide a required close pack (e.g.,a square or a hexagonal pack) for the incoming rotating elements. Forthe purposes of this application, the matrix walls include a variety ofsupporting and arranging features within the matrix, including solidwalls, walls partially patterned with channels, posts, tapered posts,etc. It is noted, that it was unexpectedly discovered that contactbetween adjacent rotating elements does not substantially affect theperformance characteristics of the rotating element displays. Therefore,in some embodiments, the walls can be reduced to the posts, which canfunction as supporting and arranging elements in the matrix. In oneembodiment, it is preferred to use a square close pack of the matrixcells, wherein the cells are defined primarily or exclusively by postswith relatively low aspect ratios (e.g., 5:1 and lower). In otherembodiments, particularly in a matrix with a hexagonal close pack, highaspect ratio walls (e.g., greater than 5:1) may be used. The choice of apacking arrangement, wall structure, and wall aspect ratio may depend ona variety of manufacturing considerations. For example, robustness ofparticular matrix material and its suitability for a hot embossingmethod, may be considered. Further, requirements for the displaycontrast and brightness should also be taken into account, when aparticular matrix configuration is chosen.

In addition to the particular shape of the matrix, some aspects of thepresent invention focus on optical properties of the matrix. In order tomaximize the contrast and brightness of the display, the reflectance ofthe interstitial area should be relatively high. This can be achieved,if the containment matrix itself (or at least the portion of the matrixdisplayed in the interstitial area between the elements) has highreflectance, preferably comparable to the lightest color of theelements. In certain embodiments, the viewable portions of the matrixhave a diffuse reflectance of at least about 35%, preferably at leastabout 50% and more preferably at least about 70%. Further, the color ofthe viewable portions of the matrix should be the same or similar tothat of the lightest color of the elements. For example, the matrix canbe white or light-yellow, light blue, light pink, etc., depending on the“light” color of the rotatable elements.

In particular embodiments, a matrix having any one or more of theproperties listed above (e.g., light colored interstitial regions) canbe used in conjunction with displays that employ translationally movingparticles. For example, particles, such as those used in the E-Ink orSipix electronic paper technologies can be confined within the cells ofa matrix as described above. In some embodiments, electrically andoptically distinct particles of two colors (e.g., black and white)similar to those used in E-Ink technology, or particles of one color(e.g., white) suspended in a liquid of a different color from the colorof the particles, similar to those used in Sipix technology, may beemployed in conjunction with the matrix described herein. In one suchembodiment, one or more particles having one color are suspended in afluid having a color different from the color of the particles. Forexample, white titanium dioxide particles may be suspended in a blackfluid. The particles are charged so as to translationally move towardsone of the electrodes when a potential is applied to the electrodes ofthe display. Depending on the applied potentials, the particles may moveeither to or from the viewable area of the display. The particles, whichare typically confined within transparent capsules are divided into anaddressable array by a matrix, so that each of the matrix cells wouldcontain one or more capsules, wherein each capsule contains a pluralityof particles suspended in a fluid. Alternatively, each matrix cell maycontain a plurality of unencapsulated particles. The electrodes addressthe cells of the matrix, so that, for example, one matrix cell maycorrespond to one pixel of the display. The pixel appears white to theviewer when white particles move towards the viewable area of thedisplay and it appears dark when the particles move away from theviewable area. These types of displays are known as electrophoreticdisplays since they involve translational movement of charged particlesin an electric field. This technology is described in detail in U.S.Pat. Nos. 5,930,026, 6,672,921, 6,067,185, 6,987,603, 6,839,158,6,727881, and 6,795,229 which are incorporated herein by reference intheir entirety and for all purposes.

Typically, in this embodiment, the matrix walls, when projected onto theviewable area of the display occupy about 5-7% of visible area whenrectangular matrix cells are used. In certain embodiments, the structureof the matrix provided for this embodiment may differ in some regardsfrom the matrix structure provided for rotating elements, since theparticles used in this embodiment are typically much smaller thanrotating elements, and one matrix cell may contain a large number ofparticles (either free or contained within microcapsules). Therefore,structurally, the matrix cells need not conform to shapes of individualparticles. For example, rectangular instead of cylindrical cells may beused. Further it may be important to localize particles withinindividual cells and to prevent their mixing during the use of thedisplay. Therefore, the cells of the matrix need not be in communicationwith each other, if such communication would allow particle mixing. Forexample, cells may be completely isolated from each other, or may havepores or channels that would not be wide enough for the particles (orcapsules) to cross from one cell to another.

In particular embodiments, it may be important to maximize reflectanceof the display by maximizing the reflectance of the matrix. In theexisting E-Ink and Sipix technologies the matrix was typically made of atransparent material, rather than of a material having high reflectance(e.g. light or white) at least in its viewable areas. In order tomaximize brightness and contrast of the display, use of a matrix havinghigh reflectance in the viewable interstitial area in an electrophoreticdisplay (such as E-Ink or Sipix electronic paper display) is desirable.

While it is advantageous to use a light colored matrix in someembodiments, in other embodiments it may not be crucial. In general, thematrix can have a wide array of optical properties, and may have variousdegrees of translucency and reflectance. For example, in someembodiments a transparent matrix may be used.

Further, for some applications, viewable interstitial regions of thematrix may have a color that is close or identical to one of the colorsof the optical element of the electro-optic display regardless whetherthis color is light or dark. For example, a dark (or black) matrix maybe used in combination with black and white bichromal spheres in certainapplications, such as when light images on dark background need to bedisplayed. This also applies for the use of the matrix in conjunctionwith translationally moving electrophoretic particles. For example,viewable portions of the matrix may be the same color as thetranslationally moving particles, or the color of the liquid in whichthe particles are moving.

The matrix, in addition to its containment and light-reflectionfunctions, also allows the cells, in some embodiments, to be in fluidcommunication with each other by providing one or several channelsconnecting the cells. Any number of individual cells can be connected bythese channels, e.g., rows of cells can be connected by parallelchannels or the matrix can contain both parallel and perpendicularchannels, such that rows and columns of cells are in fluid communicationwith one another. The fluid communication between the cells servesvarious purposes. First, it allows the display structure to be filledwith fluid through the channels in the matrix during the manufacturingprocess. This can be accomplished, after the device has already beenpartially fabricated, e.g., when the spheres have been placed into thecells and the front electrode layer has been sealed with the matrix.Such fabrication method compares favorably with other fluid-fillingmethods, since the channels allow the fluid to be easily aspirated intoevacuated front plane structure in the final stages of front planemanufacturing, e.g., through side openings of the partially fabricateddevice. Secondly, fluid communication between the cells allows thedilution of light-absorbing impurities that may leach from the matrixinto certain cells. Such impurities could lead to failure of individualpixels if the cells were isolated. Further, if optical properties of thefluid deteriorate over time, the channels would allow the fluid to beremoved and the display to be refilled by fresh fluid, if necessary.

While fluid communication between the cells can also be achieved byemploying a fluid-filled gap between the matrix and the front electrodelayer, the channel design affords a thinner display and minimizestranslational movement of the spheres within the cell, and hence,minimizes the driving voltage. The channel design also results in a morerobust structure of the display, since the top part of the matrix isattached to the front electrode layer and ensures that the front layeris equidistant from the backplane and the sphere monolayer at any point.In the case of the gap design, the matrix is not attached to the frontelectrode layer, and the columnar pressure of the fluid can lead tobowing out of the front plane, possibly causing image distortion anddecreased reliability of the device. Nevertheless, it should beunderstood that certain embodiments of the invention allow for a limitedgap between the front plane and the upper surface of the matrix. Stillother embodiments may not require a channel or a gap at all. Forexample, in some embodiments cells containing rotating elements may notbe in fluid communication with each other (isolated).

In some embodiments, it may be advantageous not to allow all of thecells in a matrix to be in fluid communication, but to divide the matrixinto several regions, such that all of the cells within the same regionare in fluid communication, while the cells from the adjacent regionsare separated by a solid wall that does not allow for fluidcommunication between the cells of adjacent regions. For example, asolid wall can divide the communicating cells every 2-100 (e.g., 5-50)rows of cells, thereby creating non-communicating regions of the matrix.The advantage of this design is that a failure of a single matrix cell,due to, e.g., leaching of a colored component into the dielectric fluid,will not lead to the failure of the entire display, but would beconfined only to the region in which the cells are in fluidcommunication, without affecting adjacent regions which are separatedfrom the failing region by a solid wall. For example, a matrixcontaining regions having a plurality of posts, wherein the posts definethe arrangement of individual cells, and wherein the regions areseparated from one another by solid walls, may be employed.

While in some embodiments the provided displays primarily make use ofthe rotational movement of the rotating elements without using theirtranslational movement, other embodiments may make use of translationalmovement of the rotating elements, in order to further increase thecontrast of the display. For example, the height of individual cells canbe increased, such that a rotating element could translationally movebetween the electrodes towards and away from the viewer, in addition toits rotational movement. In these embodiments the height of the cell canbe at least 1.1 (e.g., 1.5) times greater than the largest dimension ofthe rotating element. While this embodiment may require a greaterdriving voltage for the display, the contrast of such display is oftenimproved compared to a purely rotational display. In addition, thehigher walls are preferred during the display assembly process, becausethey provide an additional space to be occupied by an adhesive duringthe operation of attachment of the front electrode layer to the matrix.

While a containment matrix, in general, is useful for providing anordered array of spheres, in selected embodiments it may be absent fromthe design or may be substituted by alternative structures.

In certain embodiments, at least some of the rotating elements arearranged in a “monolayer.” A monolayer of rotating elements will residebetween the back and front electrodes of the display and will presentmost of the viewable elements. Generally, the monolayer will be providedas a surface layer of the display and will have a thickness that is notsubstantially greater than the dimensions of a single rotating element(or at least a cell used to constrain it). The monolayer will typicallyconform, at least generally, to the shape of the display. If the displayis flat or substantially flat, then the monolayer typically will besubstantially flat. If the display has some curvature, the monolayerwill typically have a corresponding curvature. Commonly, the monolayerwill contain closely packed rotating elements; these elements should notnecessarily form a defined geometrical structure on a surface. Forexample, a hexagonally packed layer and a randomly packed layer ofelements are both within the scope of present invention. In someembodiments, the monolayer may also include elements of smallerdimensions than those in the primary layer, located in the front of orbehind the primary layer of elements. In yet other embodiments, thedisplay device may contain additional rotating elements that maysupplement the monolayer.

While the element monolayer may be substantially planar (or otherwiseconform to the shape of the display), it should be understood that therotating element display may be flexible or even foldable in someembodiments, and therefore may have significant variation inconformation.

In yet other embodiments, one or several layers of rotating elements canbe used in conjunction with a matrix which provides cells designed tohost a plurality of elements. For example the matrix can have cellsarranged in a hexagonal or square close pack, wherein each cell may hosta monolayer or several layers (e.g., between about 2-5 layers) ofoptically anisotropic elements. For example, between about 10-100elements can reside in a single cell. Advantageously, when severallayers of elements are used, the contrast of the display can be improvedbecause the elements from the underlying layers, which rotate in concertwith the elements of the first viewable layer fill in the interstitialregions of the first viewable layer with the correct color. In someembodiments, it is advantageous to shape the walls of the matrixcontaining the plurality of the elements, such that the walls mold tothe shape of the plurality of elements residing at the wall. Forexample, for spherical elements, a matrix with scalloped walls can beused.

Rotating Elements

Elements suitable for use in the twisting element or rotating elementdisplay may have a variety of shapes and structures. Rotatable elementssuitable for use with this invention may be shaped as spheres,cylinders, ellipses, ovals, football-shaped elements, and the like.Structural aspects of these elements will be illustrated with thereference to spheres, but it should be understood that the samestructural considerations can be applied to other shapes.

A sphere used in this invention can have a hollow or solid core, and becoated with one or more coatings, so that the coating or coatingsprovide optical and electrical anisotropy to the sphere. A variety ofcoating methods known to those of skill in the art can be used. In someembodiments hemispherically coated elements can be manufactured by thetransfer coating methods described in commonly owned application Ser.No. 60/876,767, titled “Hemispherical Coating Method forMicro-elements”, naming Lipovetskaya et al. as inventors filed Dec. 22,2006, which is herein incorporated by reference in its entirety and forall purposes.

In one example, the core sphere can be hemispherically coated (orapproximately hemispherically coated) with two coatings having differentoptical and electrical properties. For instance, a white coatingproviding negative charge and a black coating providing positive chargecan be employed. The core sphere itself may be neutral or charged. Inthis example (employing two different coatings on a core sphere),optical properties of the core sphere are typically not important, sinceits surface is not presented to the viewer.

In those cases, when the core sphere itself has suitable opticalproperties (e.g., highly diffuse or specular reflectance), it can behemispherically coated with a coating differing from the core sphereboth optically and electrically. For example, a white essentiallyneutral sphere can be hemispherically coated by black pigment carryingnegative charge.

In an alternative embodiment, no core sphere is employed, and insteadtwo hemispheres with different properties can be fused together orotherwise combined to form an optically and electrically anisotropicsphere. For instance, solid or hollow black and white hemispherescarrying opposite charges can be fused.

Examples of elements that can be used in the display structure areillustrated in FIGS. 3A and 3B. FIG. 3A shows a side view of a bichromalsphere 301, and FIG. 3B shows a sectional view of the same sphere. Inthe depicted example, a hollow core sphere 303 is completely coated withan opaque white coating 305, and with a black hemispherical coating 307on top of the white coating. While black and white colors of the spheresprovide the highest contrast images, other color combinations can beused as well. Especially advantageous combinations include those inwhich one side of the sphere is substantially darker than the other,e.g., white and red, yellow and blue, yellow and black, or any otherlight and dark combination. Light and dark colors are relativeproperties. However, in certain embodiments presented herein, lightcolors are defined as those having at least about 50%, preferably above80% reflectance. In certain embodiments the black or darker color has areflectance of less than about 10%, more preferably less than about 5%,and even more preferably less than about 2%. In other embodiments, thecolors of rotating elements, need not necessarily be light and dark, butmay be complementary to each other so that a desired visual effect iscreated. For example, black and orange, black and red, white and blue orother color combinations capable of creating a well viewable image, maybe used, regardless of relative lightness or darkness of these colors.

Note that it can be difficult to measure the reflectance values of smallcurved objects such as the rotating elements employed in this invention.To address this challenge, reflectance can be measured from a large setof rotating elements in a close packed arrangement in a monolayer (e.g.,a hexagonal close pack arrangement) attached to adherent tape.Obviously, the reflectance value is function of the color of the tapebackground. The above values of reflectance may be measured using whitetape (i.e., tape providing a white background for the reflectancemeasurements). A common technique may involve measuring spectralresponse of a sample using an industry standard spectrometer, such as aGretag Mcbeth Spectroscan.

While an advantageous property of the present invention resides in theability to provide high-contrast images, it can also be used inapplications in which high contrast is not needed. For example,green/tan color combination can be used to produce an electronicmaterial with a camouflage-like appearance for military applications,and the like.

Aside from possessing different colors, the spheres can possess othertypes of optical anisotropy. For example, hemispheres with differentlevels of diffuse and specular reflectance can be used. In someembodiments one hemisphere has a high specular reflectance, while theother is opaque and dark-colored. For example, a specularly reflectiveglass core sphere can be hemispherically coated with a dark pigment,without application of a white layer.

In other examples, the rotating elements may have retroreflective orlight-emitting portions. Further, rotating elements having luminescent(e.g. fluorescent or phosphorescent) portions may also be used. These,for example, may be prepared by applying hemispherical coating dopedwith luminescent material to the core sphere.

The spheres should be relatively small in order to provide good displayresolution. Appropriate sizes include spheres with diameter ranges ofabout 25-150 μm, preferably 35-100 μm, for example about 50 μm.Obviously there may be some variance in the sizes of the spheres in agiven display. This can be controlled by appropriate sizing techniquessuch as sieving. In certain embodiments, the matrix has a cell size of50 um, while the spheres have a nominal diameter of about 47 μm with ausable range between 45 μm and 49 μm, with a standard deviation of about1 μm. For optimal optical performance, the sphericity of rotatingspheres should be high (e.g., at least about 95%), and the sizedistribution in the population of spheres should be 10, small. Forexample a population of coated spheres that range from about 45 to 49 μmin diameter can be used in an individual display. The hemisphericalcoating layer can be about 1-2 μm thick, and should not significantlydistort sphericity of the elements. In certain embodiments, the surfaceroughness of this coating does not exceed about 0.5 μm. Alternate sizingcombinations can also be implemented. For example a matrix with a cellsize of 58 μm could utilize a spheres with a nominal size of 55 μm, anda range of 53 to 57 μm.

It should be realized, that much larger elements may be used in certainembodiments. For example, for billboard signs that are typicallyviewable from the distance of several hundred feet, rotating elementshaving a diameter of 1-2 inches may be appropriate. In general, elementsof any size, that would produce a good display resolution for aparticular application of the display, will be suitable

The materials used for the core sphere and for the coatings shouldpreferably have a melting point or glass transition point of higher than100° C., in order to withstand high-temperature operations during themanufacturing process and exposure to high temperature during end use.The coating materials should also be compatible with the fluid in whichthe spheres will be suspended for rotation, e.g., they should notdissolve or swell in this fluid. Further, it may be desirable to employelements having a density that is similar to that of the dielectricfluid. In certain embodiments, the spheres, or at least the core sphereis made from a material such as glass, ceramic, or polymer. The spheres,although they can be made from intrinsically brittle materials, shouldhave good crushability characteristics. For example, they may be able towithstand compressing liquid pressure in the range of 350-3000 psi, asused in the standard industry crushability test.

Rotation of the spheres by 180 degrees (or by a greater or lesser amountas necessary to effect a display transition) should occur within adefined window of electric field. The transition should occur easily atfields generated by actuation of backplane control circuits. Typically,a field of about 0.2 V per micrometer is generated (e.g., a potentialdifferential of about 15-20 volts is generated over 75-100 micrometers(a typical back and front electrode separation distance of thisinvention) but should not occur at significantly lower voltages such asthose encountered to charge lines in the backplane. Thus, for example,display transitions may be designed to occur at fields attained at 15-20volts over 75 micrometers.

In a specific example, erasing the display image to all white colorinvolves applying 20 V to all of the back plane electrodes withreference to ITO electrode layer. Erasing the display to all black colormay involve flipping the polarity, so that ITO electrode layer is at 20V with reference to all back plane electrodes. Alternatively, erasingthe display image to all black color may involve applying −20 V to allof the back plane electrodes with respect to the ITO layer. T In thisexample, the total voltage drop in any pixel is never greater than 20V,but the bus carrying voltage must be able to handle the range of −20 Vto +20 V. The +/−20 V window is a suitable potential for organicsemiconductor/conductor backplanes. When using other types ofbackplanes, such as inorganic TFTs, for example, higher voltages (e.g.,+/−40 V) may be employed. For segmented circuit board back planes, onecould use even higher voltages. It is expected that such voltages willgenerate a torque force sufficient to rotate the sphere. However, designconsiderations require that the spheres have a mass and surface chargedistribution appropriate to accomplish this. It is, therefore, importantto provide spheres made from materials with appropriate densities, sothat they could be rotated in the designed voltage range. Depending uponthe dielectric fluid employed and other design criteria, the spheres mayhave a density of between about 0.4 and 6 g/cm³, preferably betweenabout 0.4 and 1.3 g/cm³. In certain embodiments, it may be advantageousto use hollow spheres, which may be made of glass, ceramic, orhigh-temperature resistant polymeric materials. Such spheres may havedensities ranging from 0.03-2.5 g/cm³. Solid glass, ceramic, orpolymeric spheres with densities ranging from about 1 to 6 g/cm³ canalso be used.

The core spheres with such characteristics can be obtained from variouscommercial suppliers. In some cases, these spheres are marketed forsurface processing applications. It may be necessary to sievecommercially obtained spheres in order to ensure a tight sizedistribution suitable for electrophoretic displays. For example, hollowand solid glass spheres can be obtained from 3M Corporation, Maplewood,Minn. (Scotchlite glass bubbles, K-series (e.g., K1, K15, K25, K32, orK46)). In addition, hollow glass spheres can be obtained from PottersIndustries, Berwyn, Pa. Ceramic spheres can be obtained fromSaint-Gobain Coating Solutions, Northampton, Mass., and plastic spherescan be obtained from Asia Pacific Microspheres SDN BHD, Selangor DarulEhsan, Malaysia & Grinding Media Depot, Wyncote, Pa.

The spheres should be electrically anisotropic, in order to be sensitiveto the electric field. Electric anisotropy does not necessarily implythat the two hemispheres are oppositely charged. It is sufficient, thatthere is some nonuniformity in the charge distribution about the surfaceof the sphere and that this nonuniformity correlates with opticalanisotropy. A variety of different charge distributions are possible.Some of the examples are shown in FIG. 4. As illustrated by spheres 401and 403, one hemisphere can be neutral, while the other can bepositively or negatively charged. In sphere 405, the hemispheres haveopposite charges of equal magnitude. It is also possible to haveoppositely charged hemispheres with one charge being greater than theother as illustrated by sphere 411. In sphere 407 both hemispheres arenegatively charged and higher charge density exists on a blackhemisphere. In another example, depicted by sphere 409 both hemispheresare positively charged, with higher charge density residing on a blackhemisphere. In general, spheres having any nonuniformity in chargedistribution about their surface, e.g. a dipole moment, can be used.

In one embodiment, significant quantity of charge is provided to thesphere via the coatings, particularly by the hemispherical coating,while the core sphere is neutral or possesses a small amount of charge.The hemispherical coating should preferably provide a permanent electriccharge. The necessary charge may be provided, for example, by thepigment or the binder of the coating or by special charge enhancingadditives. Examples of these additives include quaternary ammoniumcompounds, organic sulfates and sulfonates and other compounds known tothose of skill in the art, such as those listed in U.S. Pat. No.6,379,856, which is incorporated herein by reference in its entirety.The necessary charge may also be provided by special processingtechniques of coating preparation and application. These techniquesimpart charge by, e.g., applying friction to the coating material as itis dispensed.

In one embodiment, the coating is relatively thin in comparison to thedimensions of the sphere (e.g., about 1-2 μm thickness). The coatingshould provide optical and surface properties as presented above (e.g.,reflectivity, opacity, color, and roughness). In certain embodiments,the coating is made from binder and a pigment. In some embodiments, asolvent may be added for manufacturing. In some embodiments, a specialcharge-imparting agent may be added.

The spheres are electrically anisotropic when immersed into the fluidfor rotation. While it is advantageous that they possess a permanentelectrical anisotropy, in some embodiments this anisotropy may beinduced or enhanced when they are immersed into the fluid, either by thefluid itself or by other means.

The fluid should have appropriate characteristics that will allowsufficient rotation (e.g., complete 180 degree) of the sphere in thepreferred driving potential range. Dielectric fluids, essentiallynon-conductive transparent fluids, such as silicon oils, mineral oilsand isoparaffins are suitable. The fluid should preferably have aconductivity of less than 1000 femtomho/cm (femtosiemens/cm), preferablybetween about 20 and 200 femtomho/cm. In certain embodiments, the fluidwill have a low dielectric constant (e.g., less than about 2.5(preferably less than about 2)) and a viscosity of between about 0.5 and5 centistoke. These characteristics are selected so that a thresholdvoltage, response time, and operating voltage window of the device areoptimized. Examples of suitable dielectric fluids include siliconefluids, such DC200 available from Dow Corning of Midland, Mich.,isoparaffins, such as ISOPAR, available from Exxon Mobile of Irving,Tex., and fluorinated fluids manufactured by 3M.

Throughout this document, hemispherical optical and electricalproperties are discussed. This does not imply that the optical orelectrical properties are limited to exactly hemispherical dimensions.In certain embodiments, the elements may be designed to have one opticalmaterial occupy less than a full hemisphere of the element's surface andanother optical material occupy more than a full hemisphere. In someembodiments, it may be advantageous to have more than two opticallydifferent portions within one sphere. For example a sphere having twoless than hemispherical coatings (e.g. opaque black and white) with atransparent strip separating them may be useful for some applications.The strip will provide a clear transmissive effect that is verydifferent from normal black and white appearance. Examples of elementsof this type are described in U.S. Pat. No. 5,892,497, which is hereinincorporated by reference in its entirety.

Further, even if the desired result is hemispherical, it should be notedthat suitable displays may be produced, in certain embodiments, usingpopulations of rotating elements that have significant variance in thegeometric extent of the optical or electrical properties. For example,it may be suitable to use a population of rotating elements havingoptical coatings that vary on average by 10% or even 20% from aperfectly hemispherical covering. Other applications may not toleratesuch wide variance.

Matrix Design and Fabrication

As indicated, displays of this invention may employ a matrix or otherstructure for confining rotating elements. The structure of a typicalmatrix allows containment of equally spaced rotating elements in anordered array of cells or cavities. In one embodiment, the cells arearranged in a hexagonal geometry (e.g., a hexagonal close pack pattern)with minimal distance between the centers of adjacent cells. Otherembodiments of the matrix structure, such as those providing square,rectangular or rhomboidal arrays for packing of the spheres, can beused. In general, the matrix can provide a containment structure of anydesired geometry for the spheres as well as for the elements of othershapes (e.g., cylinders, football-shaped elements and the like). In oneof the depicted embodiments, the cells are defined in part by serpentinewalls offset from one another to allow a hexagonal packing arrangement.In other embodiments, other wall designs (e.g., straight or relativelyuncurved shapes) or even pillars (posts) may be employed.

A number of exemplary matrix configurations will be described. Thematrix configurations include but are not limited to configurations withand without channels; configurations with square, hexagonal, andrhomboidal close packs of cells, configurations with continuous walls,walls with channels formed therein, and posts; configurations where eachcell is designed to host one or plurality of rotating elements,configurations where the matrix allows for purely rotational movement ofthe elements and configuration in which the matrix design allows fortranslational movement of rotating elements between the electrodestowards and away from the viewer. The matrix configurations are notlimited to the described examples, and a variety of alternative matrixconfigurations can be used, as will be recognized by one of skill in theart.

A matrix configuration with an arrangement of cells in a hexagonal closepack and with a number of parallel channels extending through an entireheight of the cell wall, is illustrated in FIGS. 5A-5B. FIG. 5A providesan isometric view of one example of a matrix 501 in accordance with thisembodiment. A top view of this matrix is shown in FIG. 5B. The matrix501 includes a matrix base 503 and matrix walls 505. A series ofparallel channels, including a channel 507, run through the matrixwalls, and provide fluid communication between the cells 509. In thedepicted embodiment, the cell walls are arranged so that a hexagonalclose-pack structure is provided for the spheres. The cells have anessentially cylindrical geometry, with a diameter of the cell being onlyslightly greater than the diameter of the sphere. The depth of the cellsis defined by the height of cell walls and in this embodiment does notsubstantially exceed the diameter of the spheres. In alternativeembodiments, higher walls may be employed, and the matrix design mayallow for a translational movement of the spheres within the cells.

Generally, it is advantageous to closely match dimensions of the holdingcell to the dimensions of the element that is disposed within it. Theinternal cell dimensions (typically height and width or height anddiameter) should be selected preferably so that the element does notrotate until a certain threshold voltage is applied to the displayelectrodes. Together with the carefully selected viscosity of thedielectric fluid, the matched dimensions of the cell and the element areimportant factors for providing a desired voltage window and displaybistability. Further, in those embodiments, where purely rotationalmovement is desired, the cell dimensions should be chosen to not allowsubstantial translation of the element within the cell. In certainembodiments, the cell dimensions should exceed the dimensions of therotating elements by only about 1 to 10 μm, more preferably by about 1to 5 μm. For example, for spheres with diameters ranging from about 45to 49 μm, cells with a diameter of about 50-55 μm and a depth of about50-55 μm may be employed.

By controlling the cell height to provide minimal separation between theback and front electrodes, one minimizes the driving voltage necessaryfor display operation. For a display employing spheres with a nominaldiameter of approximately 49 μm, a separation between front and backelectrodes of about 75 μm can be achieved with the base of the matrixhaving a thickness of about 25 μm, and the height of the cells of about50-55 μm. Obviously, these dimensions can be scaled for other spheresizes.

As indicated, it is often desirable to closely pack the rotatingelements, leaving very little room between neighboring elements. To thisend, the walls of the matrix may be made very thin, particularly atpoints where the elements come closest to one another. In certainembodiments, the minimum wall thickness is no greater than about 45 μm,e.g., no greater than about 5 μm. The center-to-center separationbetween the cells may be in the range of about 27-200 μm depending onthe size of rotating element. For example, for spheres having a nominaldiameter of about 49 μm, a center-to-center separation of about 53 to 60μm (55 μm in a typical case) may be employed.

Of course, the walls of the matrix must be sufficiently high to provideseparation from the front electrode and allow rotation of the elements.This means that the matrix may have relatively high aspect ratio walls.Walls with an aspect ratio of greater than 5:1 (height to width),preferably greater than 8:1, for example greater than 10:1 may berequired. Fabrication techniques for producing such structures aredescribed below. In the exemplary embodiment presented here, the aspectratios correspond to a wall thickness of less than about 7 μm, e.g.,less than about 5 μm at a wall height of 50-55 μm. For example, a 55 μmcenter-to-center separation between the closest spheres in the pack isachieved when the wall separating the adjacent cells has a thickness ofless than about 5 μm, e.g. less than about 3 μm. These dimensions areapplicable to an embodiment in which the diameter of the spheres rangesfrom about 45 to 49 μm. The matrix can be scaled in accordance with thesize of the spheres in use, while maintaining close spacing between thespheres, as will be understood by those skilled in the art. For example,for spheres having a diameter of about 150 μm, walls that define cellsof the matrix may have a minimum width at regions separating adjacentcells of at most about 45 μm.

It should be understood that the invention is not limited to embodimentsin which the matrix walls provide the entire internal height of the cell(in the direction between the front and back electrodes). Somestructures may employ front electrode structures or intermediatestructures that provide at least some of the internal cell height. Inone alternative embodiment, the matrix walls provide only a fraction ofthe total height required for separation and a structure associated withthe front electrode provides a mating lattice providing the remainder ofthe needed separation height. Of course, the matrix and front electrodestructures would have to be carefully aligned to provide the requiredcell structures.

As explained, some embodiments of the invention employ one or moreintercell channels to provide fluid communication between certain cells.Generally, a channel or channel network running through the walls of thematrix can run though any number of cell walls and can connect anynumber of cells with each other. As shown in FIG. 5A, the channel 507runs through two walls of each cell, and connects parallel lines ofcells in a row-like fashion. Other embodiments employing more tortuouschannel paths are possible. Other embodiments employing intersectingchannel paths are also possible. The cell walls in regions through whichthe channel runs can be entirely or substantially absent, as in theembodiment illustrated by FIGS. 5A and 5B, or, they can be present tosome extent as illustrated in a separate matrix embodiment presented inFIGS. 6A, 6B, and 6C. FIGS. 6A and 6B show an isometric view and a topview of a matrix structure in accordance with this embodiment,respectively. FIG. 6C shows a cross-sectional side view of an individualcell of the matrix. The elements of the matrix are numbered analogouslyto the elements of FIG. 5. As depicted, a channel 607 occupies the upperportion of the cell walls and has a cross-sectional area sufficient toallow free flow of fluid from cell to cell (e.g. an area of at leastabout 200 μm²). For example, this channel may have a depth of about 10μm and a width of about 20 μm. In general, the channel may occupy aparticular percentage of the cross-sectional area of the cell. Forexample, in some embodiments, it is preferable that the cross-sectionalarea of the channel is at least about 5% or at least about 8% of thecross-sectional area of the cell. Specifically, a 200 μm² channeloccupies about 8% of the cross-sectional area of a 50×50 μm cell. Forsmaller cells, smaller channels will generally be used. For examples,channels with a cross-sectional area of at least about 30 μm² may beappropriate in some embodiments. The matrix of an embodiment shown inFIG. 6A-6D also has a slight narrowing 611 at the bottom portion of thecell (e.g., at the bottom 25 μm of the cell). This narrowing typicallyfollows the contours of the sphere, therefore affording a better fitthan a strictly cylindrical shape of the cavity. The diameter of thecell, in this embodiment, refers to the diameter, measured above thisnarrowing. The embodiment of FIGS. 6A through 6C provides a strongermatrix lattice in comparison to the embodiment of FIGS. 5A and 5B, atthe expense of more limited intercell fluid communication.

While theoretically the hexagonal close pack provides the highestpossible fill factor, it also demands the highest aspect ratio for itswalls. While for some matrix sizes and for some matrix materials,matrices having walls with aspect ratios of greater than 5:1 can beeasily manufactured, for particularly small-scale matrices made offragile materials, such high aspect ratio walls may be difficult tomanufacture. Further, high aspect ratio walls may be unstable duringdisplay assembly and end use of the display.

In an example illustrated by FIGS. 7A-7C, a matrix with a square closepack arrangement of cells is provided. Advantageously, such matrix canemploy relatively low aspect ratio posts serving as its cell walls anddefining the arrangement of spheres in the matrix. For example, theaspect ratio of such posts can be 5:1 and lower. FIG. 7A provides anisometric view of a matrix structure in accordance with this embodiment.FIG. 7B provides a top view and FIG. 7C provides a cross-sectional viewof this structure respectively. The numbering of matrix elements isanalogous to the matrix structure shown in FIGS. 5A-B. It can be seenthat in the embodiment presented in FIGS. 7A-7C the matrix includes anumber of posts 705 defining the square close pack arrangement of thecells. Each cell 709 is in fluid communication with all of its adjacentcells through intersecting channels 707. Example dimensions for a matrixdesigned to host spheres having a diameter of about 50 μm are thefollowing: cell spacing of about 55 μm, and cell wall height of about 65μm. In one embodiment, the walls (e.g., posts) that define cells of thematrix have a minimum width at regions separating adjacent cells of atmost about 35% of the diameter or width of the cells. For example, postshaving widths of about 20 μm can reside between 58 μm-wide cells.

It is noted that spheres when residing in such matrix can contact oneanother through the channels, and such contact does not substantiallyaffect the performance of the display. The matrix configurationpresented in FIGS. 7A-7C has several advantages. First, the presence ofposts rather than extending walls minimizes the interstitial area of thedisplay and minimizes interactions between the spheres and the walls.Further, the square pack design according to this embodiment allows ahigh sphere fill factor. For example, a fill factor of at least about65% has been practically achieved for spheres ranging in 45-53 Mm indiameter with a cell spacing of 55 μm and a cell clearance of 58 μm.Further, such matrix can include low aspect ratio posts, which makessuch matrix more manufacturable in comparison with matrix configurationswhich require high aspect ratio features.

A variation of a matrix configuration with a square pack design isillustrated in FIGS. 8A-8C. In this variation the posts defining thecell arrangement are tapered, such that they have a larger diameter atthe bottom and a smaller diameter at the top. FIG. 8A provides anisometric view of a matrix structure in accordance with this embodiment.FIG. 8B provides a top view and FIG. 8C provides a cross-sectional viewof this structure along the A-A axis respectively. The numbering ofmatrix elements is analogous to the matrix structure shown in FIGS.5A-B. The example dimensions of matrix elements are given for sphereshaving a diameter of about 50 μm. As it can be seen from FIG. 8C thecells defined by the tapered posts have a bottom diameter of about 50.2μm and a top diameter of about 66.2 μm. The posts can be tapered atabout 7°. The design with tapered posts improves the manufacturabilityof the matrix and can offer a higher contrast, especially when a matrixwith a high reflectivity (e.g., white matrix) is used.

A number of alternative designs can be implemented in combination withthe square pack design as described above. As it will be understood byone of skill in the art, the matrix pattern can be scaled to a sphere ofdesired size, as necessary. Further in one embodiment higher walls (e.g.walls having height of at least about 1.5 of sphere diameter) can beused. As it was described, such design may allow for translationalmovement of the spheres resulting in higher display contrast. Further,higher walls can allow a top seal display assembly process, in which theadhesive envelopes the top of the post during and after the attachmentof the front electrode layer. According to another embodiment, thematrix can be segmented into several regions which are not in fluidcommunication with one another. For example, a solid wall may be placedin the matrix between about every 5-50 rows.

A matrix configuration employing a hexagonal close pack design without achannel is illustrated in FIGS. 9A-9C. FIG. 9A provides an isometricview of a matrix structure in accordance with this embodiment. FIG. 9Bprovides a top view of the matrix and FIG. 9C provides a cross-sectionalview of an individual cell in this structure. The numbering of matrixelements is analogous to the numbering used in FIGS. 5A-5B. The absenceof a channel in this design allows the walls of the structure to extendall the way to the top of the front electrode layer. This designmaximizes the wall area and results in increased structural stability ofthe formed device. Further, this design provides a large wall areaavailable for attachment of the front electrode layer and, therefore,provides a more robust device. In addition, when the matrix with highreflectivity is employed (e.g., a white matrix), this design provides avery bright display, which is typically superior in brightness to thedisplays based on matrix designs employing a channel.

In an example embodiment the matrix designed to host spheres having adiameter of about 50 μm, has a closest center-to-center cell separationof about 66 μm, a cell height of about 60 μm. The cells may be taperedwith the largest cell diameter of about 61 μm at the top of the cell. Itis understood that the dimensions can be scaled and modified toaccommodate spheres having different sizes. Further, this design may bemodified in a variety of ways, e.g., higher cell walls may be employedto enable translational movement of the spheres.

A matrix configuration employing a close pack square design withmulti-height posts is illustrated in FIGS. 10A-10C. FIG. 10A provides anisometric view of a matrix structure in accordance with this embodiment.FIG. 10B provides a top view of the matrix and FIG. 10C provides across-sectional view of the matrix structure. The numbering of matrixelements is analogous to the numbering used in FIGS. 7A-7B. In addition,instead of same-height posts, the structure includes tall supportingposts 1013 and short arranging posts 1015. The function of supportingposts is to serve in the attachment of the matrix to the front electrodelayer and to thereby support the structural integrity of the formeddevice. The function of the arranging posts is to define the arrangementof the cells, such as to define a square pack design. While it isimportant to have a certain fraction of the tall posts to maintainstructural integrity of the device, it is also advantageous to haveshort posts, because the short posts minimize the interaction betweenthe walls and rotating spheres. Further, a matrix having a fraction ofshort posts exhibits a more facile fluid communication, which is oftendesirable in the display manufacturing process. As it can be seen fromFIG. 10A, this structure allows for free fluid communication between thecells through the intersecting channels 1007. In some embodiments thedisplay structure contains at least about 30%, e.g., at least about 50%of short arranging posts 1015. The short posts can have a height of lessthan about 0.8 of the sphere diameter, preferably a height equal toabout a sphere radius and less. In general, the short arranging postscan have any height which allows them to serve as arranging elements inthe matrix, e.g., direct the spheres into a particular packingarrangement.

In an example embodiment the matrix designed to host spheres having adiameter of about 50 μm, has a closest center-to-center cell separationof about 55 μm, a cell height of about 65 μm, defined by the height ofthe tall posts, and a cell barrier height of about 30 μm, defined by theheight of short posts. It is understood that the dimensions can bescaled and modified to accommodate spheres having different sizes.Further, this design may be modified in a variety of ways, e.g., highercell walls may be employed to enable translational movement of thespheres, the matrix may be segmented into regions having no fluidcommunication with one another, etc.

A matrix configuration employing a combination of hexagonal and squarepacks is illustrated in FIGS. 11A-11C. FIG. 11A provides an isometricview of a matrix structure in accordance with this embodiment. FIG. 11Bprovides a top view of the matrix. The numbering of matrix elements isanalogous to the numbering used in FIGS. 7A-7B. In addition, instead ofidentical posts and identical cells, the structure includes square packposts 1117 defining square pack cells 1121 and hexagonal pack posts 1119defining hexagonal pack cells 1123. The matrix is designed to havealternating regions of square pack cell arrays and hexagonal pack cellarrays. Because of such combination, this matrix design can achieve highstructural stability characteristic of a square pack, and high fillfactor characteristic of a hexagonal pack.

In an example embodiment the matrix designed to host spheres having adiameter of about 50 μm, has a closest center-to-center cell separationof about 55 μm both in the hexagonal pack region and in the square packregion. This matrix design can be scaled and modified as necessary. Inone exemplary modification of this design the arranging hexagonal packposts 1119 are eliminated from the structure, such that a plurality ofspheres can arrange themselves, preferably in a hexagonal pack withoutthe help of the posts. Only the square pack arranging posts would beleft in this structure. In some embodiments the matrix may be segmentedinto regions (e.g., hexagonally packed regions and regions with squarepacking) which may be separated from one another by walls to preventfluid communication between the regions.

A matrix configuration employing cells designed to host multiple spheresin a cell is illustrated in FIGS. 12A-12D. In the illustratedconfiguration the matrix walls are arranged in a hexagonal design. Inother embodiments, the walls may be arranged in other designs, e.g., insquare, rectangular and rhomboidal designs. FIG. 12A provides anisometric view of a matrix structure in accordance with the hexagonaldesign embodiment. FIG. 12B provides a top view of this matrix. FIG. 12Cprovides a detailed view of a matrix portion. FIG. 12D provides across-sectional view of a matrix portion. The numbering of matrixelements is analogous to the numbering used in FIGS. 7A-7C. In thedescribed embodiment each matrix cell 1209 can host a plurality ofspheres. For example between about 3-1000 spheres can occupy a singlecell. Because of the absence of the walls between individual elementsresiding within one cell, a very high fill factor can be achieved bythis design (e.g., at least about 65%). The spheres may be distributedin a monolayer, or multiple layers of spheres (e.g., between about 2-5,preferably 2-3 layers) may be formed. It is often advantageous to use amulti-layer configuration, because the presence of underlying layersunder the first viewable layer can fill in the interstitial regionsbetween the spheres of the first viewable layer, thereby resulting inmore saturated colors. In the embodiment illustrated in FIGS. 12A-12D,the walls of the matrix are shaped so as to mold to the shape of thespheres residing against them. Such “scalloping” of the walls reducesthe interstitial area occupied by the walls and results in a higher fillfactor.

In an example embodiment illustrated by FIGS. 12A-12D, the matrixdesigned to host spheres having a diameter of about 50 μm, has a cellwidth of about 446 μm, as shown in FIG. 12B. The walls of each cell arescalloped so as to accommodate about 5 spheres against each cell wall ina single layer. The height of the walls is about 150 μm, allowingaccommodation of 1-3 layers of spheres per cell. The walls havethickness of about 65 μm at positions of their maximum thickness, as canbe seen in the detailed view shown in FIG. 12C. It is understood thatthe described dimensions are provided as a specific example, and are notintended to limit this matrix design in any way. The dimensions ofcells, the type of pack, and other parameters of the matrix can bescaled or otherwise modified as desired. For example, while theillustrated example shows a matrix which does not have a channel, otherembodiments of this design may have a plurality of channels (parallel orintersecting) allowing fluid communication between the cells.

One aspect of the present invention relates to the optical properties ofthe matrix. The matrix, as mentioned above, should preferably haveoptical properties resembling the properties of the “lighter” hemisphereof the rotating spheres, at least in the viewable region. Therefore, thematrix can be made of, or can be coated with, a light-colored material.In one embodiment, the matrix is coated with the same pigment or othermaterial that imparts the white color to the rotating elements. Thismaterial may apply to the entire matrix or just a portion of it such asthe front facing portions of the matrix (e.g., the tops of the walls andthe bottoms or floors of the cells). Coloring the cell floors of thematrix may be particularly important in designs where there areinterstitial regions between the cell walls and the regions covered bythe rotating elements. For a display employing white hemispheres on theelements, the matrix color is preferably a highly reflective whitematerial. In alternative embodiments, the matrix may be transparent oreven dark, or may match any color or optical property presented by theoptical element to the viewer. For example, it may have a blue colorwhen used in conjunction with blue and yellow rotating sphere. For most“paper” applications, however, it will be desirable to provide a maximumamount of whiteness or brightness in the display as viewers find this tobe the most comfortable viewing medium. In certain embodiments employinga light or reflective matrix as described here, the overall displayfront plane will have a white reflectance value of at least about 30%,more preferably about 50% and even more preferably about 70%. Note thata high value of white reflectance may be an important feature as one ormore color filters may be provided in front of the front plane in atypical application.

As with all of the materials of the display, the matrix material shouldbe heat-resistant, having a melting point or glass transition point ofat least 100° C. Further, its optical properties should not besubstantially affected by temperature in the preferred temperaturerange. The matrix material should also be essentially non-conductive.Those portions of matrix material that come into contact with thedielectric fluid should be resistant to it, so that the properties ofboth the matrix and the fluid are not substantially altered during theirlong-term contact. Generally, the matrix material should be reasonablyflexible to afford easy manufacturing and allow rugged treatment duringend use. Generally, it should have a hardness that allows it towithstand normal use but should not be too brittle to impactmanufacturability. In certain embodiments, the material will have adurometer of between about 40 shore A and 75 shore D. A variety ofmaterials with these characteristics can be used for matrix fabrication.For example, the matrix can be fabricated from materials used in hotembossing fabrication technique. Examples of these materials includeheat resistant acrylic polymers (e.g. polymethylmethacrylate (PMMA)),polyethylene terephthalate (PET), poly(ether ether ketone)s (PEEK),acrylonitrile butadiene styrene (ABS), polystyrene, polypropylene,polyetherimide (PEI), cyclo-olefin polymers (e.g. a cyclo-olefin polymersold under the trademark ZEONOR® available from Zeon Chemicals ofLouisville, Ky.), or Ultem available from GE Plastics of Pittsfield,Mass., and UV-curable epoxy. In one embodiment, polycarbonate (PC) is apreferred material. As indicated, such materials may include or becoated with a white pigment, such as TiO₂.

Hot embossing technique is especially suitable for matrix fabrication,since it allows fabrication of high aspect ratio features on the scaleof several microns or even nanometers (nano-embossing). Such a techniquecan provide a matrix with high aspect ratio walls, a feature that is noteasily attainable by other microfabrication techniques. In hotembossing, a mold is first manufactured by standard photolithography anddry etching techniques, such as those employed in the semiconductorindustry. The matrix mold is the negative image of the matrix. Theactual matrix is embossed when the molding force is applied to thematrix material under high temperature. A particular range of forces andtemperatures depends on a particular material that is used. In oneembodiment, embossing is performed at a temperature which is slightlyhigher (e.g., 10-40° C. higher) than the glass transition temperature ofthe polymeric material being embossed. For example, when polycarbonateis used as a matrix material, hot embossing can be conducted at atemperature of about 180° C., which is slightly higher than the glasstransition temperature of polycarbonate (T_(g)=150° C.), and at apressure of about 400-500 psi, e.g., about 450 psi. The polymericmaterial can be subjected to high temperature and pressure for secondsto several minutes, during which time it flows and adopts a shape of themaster mold. Higher pressure and temperature will typically shorten theembossing time. As indicated, this technique allows fabrication offeatures having high aspect ratios (e.g. higher than about 8:1) anddimensions of several microns (e.g., 5-10 em). The matrix material canbe brought into contact with the embossing mold by many differenttechniques such as a roll to roll technique, or die set. Alternately aninjection molded embossing can be implemented utilizing standardinjection molding plastics machinery.

Display Assembly Methods

The display structure can be assembled, for example, by followingprocess flow diagrams shown in FIGS. 13A and 13B. According to FIG. 13A,in the first operation, 1301, the matrix is fabricated by an embossingtechnique. The matrix can be embossed, for example, by hot embossing orby UV embossing. The matrix is embossed to provide a matrix cell depththat is greater than the diameter of a sphere that the cell will host.

In the next operation 1303, the top portion of the matrix is covered bya heat-activated adhesive, by, for example, a roll transfer method. Thestructure may then be thoroughly dried, and loaded with spheres in anoperation 1305. In one example, the spheres are dispensed onto the topof the matrix in excess and are loaded into the cells of the matrix withthe use of vibration. Other methods of loading the spheres will bedescribed in detail below. After the spheres have been loaded, the frontelectrode layer, such as an ITO PET layer with an anti-glare coating, isbrought into contact with the adhesive-coated matrix and is attached tothe top of the matrix utilizing the heat activated adhesive, as shown byoperation 1307. This operation typically requires heating of thepartially fabricated structure to a temperature of at least about 70°,e.g., such as 140° C. Next, in an operation 1309, an adhesive with arelease liner is attached to the back of the matrix. This adhesive willserve to attach the matrix to the backplane of the display. Apressure-sensitive adhesive (PSA) is typically used between the linerand the matrix base. The release liner can be removed, when necessary,to expose an adhesive-covered bottom portion of the matrix base, so thata backplane could be attached to it when needed. When attached, therelease liner protects the adhesive layer, and does not allow it tointerfere with other operations of the process. It is noted thatattachment of the adhesive-coated release liner can be performed atvarious stages during display fabrication and may depend on a particularmatrix fabrication process. For example, when the matrix is made from aUV curable material, which is subject to UV embossing, the embossingoperation 1301 can be performed by embossing a matrix on top of anadhesive-covered release liner. In those embodiments, when a hotembossing method is used, the release liner with adhesive is preferablyattached after the matrix has been embossed to avoid exposure of theadhesive to high temperature.

After the release liner has been attached in operation 1309, thepartially fabricated structure can be cut to a desired size. Then, thestructure is filled with a dielectric fluid in an operation 1311. Thefluid is introduced through side openings into the matrix channels, inthose embodiments where the matrix has a channel. The partiallyfabricated front plane structure can be first evacuated, and the fluidcan be aspirated into the structure when the vacuum is released. Thesides of the structure are then sealed, affording a completelyfabricated front plane of the display. At this point ultrasound may beapplied to the front plane structure to release the spheres which may beattached to the walls of the cells due to the presence of electrostaticcharge. For those embodiments, in which the matrix does not have achannel the cells are filled with the fluid prior to operation 1307,e.g., after the spheres have been loaded. The front electrode layer isthen attached as described above, and the structure is sealed. After thesealed front plane of the display is formed, the release liner can beremoved and any industry-standard or specialized backplane can beattached to the base of the matrix, as shown in operation 1313, therebyproviding a finished display product. It is noted that a particularsequence of operations shown in FIGS. 13A and 13B need not necessarilybe followed, and alternative sequences of operations may be performed,or some operations may be added or omitted.

An alternative method of display assembly is illustrated by the processflow diagram presented in FIG. 13B. This process differs from theprocess described above in that the heat-activated adhesive is appliedto the front electrode layer, rather than to the top of the matrix. Thisis more advantageous because the presence of adhesive on the matrix caninterfere with the sphere loading process. Thus, in some embodiments, itis advantageous to load spheres into the clean matrix in order to avoidcontamination with the adhesive. According to the process flow shown inFIG. 13 B, the matrix is fabricated by embossing the desired matrix inan operation 1315. The top portion of the embossed matrix containingcell openings is clear of adhesive and is exposed. The spheres are thenloaded into the matrix cells in an operation 1317. In an independentoperation 1319, which may be performed before or after operations1315-1317, the front electrode layer is coated with a heat-activatedadhesive. For example, an optically clear adhesive can be laminated ontoITO coated PET. Next, in an operation 1321, the adhesive-coated frontelectrode layer (e.g., ITO PET) is attached to the top portion of thematrix by applying heat to the matrix/electrode assembly which activatesthe adhesive. Then, an adhesive with a release liner is attached to theback portion of the matrix in an operation 1323 and the partiallyfabricated display is cut to a desired size. The structure is thenfilled with a dielectric fluid in an operation 1325 and the edges of thematrix are sealed to contain the fluid. After the cells are filled withfluid, ultrasound may be optionally applied to release the spheres fromthe cell walls. Next, in an operation 1323 the release liner is removedand a backplane is attached to the base of the matrix to provide adisplay structure.

One of the challenging operations of the display assembly process is theoperation of loading the spheres into the cells. Because of the smallsize of the spheres (typically less than 1 mm in diameter), small sizeof the matrix cells, and the tendency for aggregation exhibited by smallspheres, it is often difficult to load the spheres into the matrix bydry loading methods. According to one aspect of the invention, a methodfor loading spheres into the matrix is provided. An example processdiagram for this method is shown in FIG. 14. The process starts bypreparing a slurry of spheres in a fluid in an operation 1401. Forexample, the same dielectric fluid which will be later used to fill inthe cells, may be employed, to minimize contamination. Next an absorbentmember (e.g., a brush) is loaded with the slurry in an operation 1403.It is often preferable, but not always required that the brush materialis non-electrostatic and does not significantly change the charge of thespheres. While the use of brush is often preferable, other absorbentmembers, such as sponges may be used in some embodiment. The spheres arethen transferred to the top of the matrix by contacting the matrix withthe loaded absorbent member (e.g., brush) in an operation 1407. Thespheres are then loaded into the matrix cells by moving the absorbentmember against the matrix in an operation 1407. Next, excess spheres areremoved from the top of the matrix in an operation 1409, e.g., byswiping the spheres off with a second absorbent member (e.g., a secondbrush). The provided method achieves efficient transfer of spheres intothe matrix cells and can be used to fill any matrix type, includingmatrices that host one sphere per cell, and matrices designed to hostmultiple spheres in a cell.

The methods illustrated in FIGS. 13A-B and 14 describe fabrication ofelectro-optic displays having new types of front plane configurations.These front planes can be used in conjunction with a variety of backplanes, as will be appreciated by one of skill in the art.

In one embodiment, the backplane is provided on a flexible substrate. Insome cases, it will be desirable to employ backplane circuitryfabricated at least in part using solution phase processing. In otherwords, the conductive lines and/or the active switching elements (e.g.,transistors and/or diodes) are formed from materials deposited insolution phase. Such materials may be printed on a backplane substrate.As explained in US Published Patent Application No. 2004-0179146, filedJan. 16, 2004 (which is incorporated herein by reference for allpurposes), at least some of the conductive or semiconductive materialsemployed in a backplane may be doped or undoped organic materials suchas a polyacetylene, a poly(phenylene), a poly(phenylene vinylene), apolyfluorene, a polythiophene, a polycyclopentadithiophene, apolyaniline, a poly(ethylenedioxythiophene), and a polypyrrole.

Overall, the front plane of the display can have a thickness of lessthan about 300 μm, preferably less than about 150 μm, while the completedisplay structure which includes the backplane can have thickness ofbetween about 250 μm and 500 μm. This thickness is comparable to thethickness of paper, and allows the display to be flexible andlightweight.

Method of Use

The rotating element display is suitable for displaying both still andmoving images. The images are created by providing signals to displayelectrodes (e.g., back plane electrodes) in the addressable electrodematrix. The signals can selectively address specific electrodes, whereineach electrode allows independent control of a discrete region of thedisplay. For example, each electrode can control one or multiplerotating elements. A potential difference between the front and backelectrodes is created in response to the signal, causing the addressedrotating elements to flip and change the pixel color presented to theviewer.

The signals can control the magnitude and polarity of the voltageapplied to individual electrodes, as well as the duration of electricalpulses. Therefore, the signals can addressably control the response ofindividual elements or groups of elements on the display. For example,the signal can determine whether the addressed element should rotate ornot, and if it should rotate, the degree of rotation may be alsocontrolled by the provided signal, utilizing pulse width modulation(PWM). Still and moving images can be thus created on the rotatingelement display. In one embodiment, the inventive display possessesbistability. In this embodiment, still images can be maintained on thedisplay without change in the absence of electrical input fromelectrodes or other signals.

Other Embodiments

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art.Although various details have been omitted for clarity's sake, variousdesign alternatives may be implemented. Therefore, the present examplesare to be considered as illustrative and not restrictive, and theinvention is not to be limited to the details given herein, but may bemodified within the scope of the appended claims.

For example, while it is advantageous for some applications thatelectro-optic displays possess bistability, in other embodiments (e.g.,in embodiments directed to video applications), the displays of presentinvention may not necessarily be bistable. Further, while reflectivedisplays were primarily described in the examples provided in thedetailed description, it should be realized that the displays of presentinvention are not limited to reflective displays and can also includedisplays that may be transmissive, may employ back-light or employlight-emitting elements.

1. A rotating element display comprising: (a) a back plane comprising aplurality of electrodes distributed in two dimensions on the backplane,wherein each electrode allows independent control of a discrete regionof the display; and (b) a front plane comprising: a first side connectedto or proximate said back plane; at least one electrode on a second sideof the front plane opposite said back plane; a matrix comprising (i) aplurality of cells defined by walls in the matrix and (ii) at least onechannel through at least some of the walls and connecting at least someof the cells with one another; a plurality of optically anisotropicelements disposed in said plurality of cells; and a fluid provided insaid cells, such that said elements can rotate from a first orientationto a second orientation within their respective cells when an electricfield is applied to the cells.
 2. The display of claim 1, wherein theanisotropic elements are hemispherically coated elements.
 3. The displayof claim 1, wherein the matrix comprises a plurality of channels eachconnecting a plurality of cells along a path.
 4. The display of claim 3,wherein the plurality of channels are substantially parallel to oneanother
 5. The display of claim 3, wherein the channels are arranged toallow the fluid to be drawn into the front plane during assembly of thefront plane.
 6. The display of claim 1, wherein at least some of thematrix walls are posts.
 7. The display of claim 1, wherein at least onecell in the matrix is in fluid communication with each of its adjacentcells.
 8. The display of claim 7, wherein the matrix comprises aplurality of regions, wherein the cells within the same region are influid communication with one another, and the cells from differentregions are separated from one another by a wall preventing fluidcommunication between adjacent regions of the matrix.
 9. The display ofclaim 8, wherein each region comprises between about 2-100 rows ofcells.
 10. The display of claim 1, wherein the matrix comprises wallshaving different heights.
 11. The display of claim 10, wherein thematrix comprises supporting walls and arranging walls, wherein thesupporting walls have a height of at least equal to a height or adiameter of an optically anisotropic element, and wherein the arrangingwalls have a height of less than 80% of the height or a diameter of anoptically anisotropic element.
 12. The display of claim 1, wherein theheight of the cells allows for a translational movement of an opticallyanisotropic element in a cell in a direction defined by the first andsecond sides of the front plane.
 13. The display of claim 1, wherein aheight of a cell is at least about 1.1 times greater than a height or adiameter of an optically anisotropic element residing in the cell. 14.The display of claim 1, wherein one optically anisotropic elementresides in a cell.
 15. The display of claim 1, wherein more than 1optically anisotropic element resides in a cell.
 16. The display ofclaim 15, wherein one layer of optically anisotropic elements resides ina cell.
 17. The display of claim 15, wherein between about 2-5 layers ofoptically anisotropic elements resides in a cell.
 18. The display ofclaim 1, wherein the fluid is a dielectric fluid.
 19. The display ofclaim 1, wherein an area projected by the cells on the first side of thefront plane occupies at least about 65% of a corresponding area of thefirst side.
 20. The display of claim 1, wherein the walls that definecells of the matrix have a minimum width at regions separating adjacentcells of at most about 13% of the diameter or width of the cells. 21.The display of claim 1, wherein the walls that define cells of thematrix have a minimum width at regions separating adjacent cells of atmost about 35% of the diameter or width of the cells.
 22. The display ofclaim 21, wherein the walls, at positions of their greatest height andleast width, have an average aspect ratio of less than about 5:1 (heightto width)
 23. The display of claim 1, wherein the walls, at positions oftheir greatest height and least width, have an average aspect ratio ofat least about 5:1 (height to width).
 24. The display of claim 1,wherein the walls, at positions of their greatest height and leastwidth, have an average aspect ratio of at least about 8:1 (height towidth).
 25. The display of claim 1, wherein the optically anisotropicelements have at least two colors, and wherein when said elements are inthe first orientation one color is presented to the first side of thefront plane and when said elements are in the second orientation adifferent color is presented to the first side of the front plane. 26.The display of claim 25, wherein the at least two colors are black andwhite.
 27. The display of claim 25, wherein one color of the at leasttwo colors is lighter than a second color of the at least two colors.28. The display of claim 25, wherein at least a portion of the matrixvisible through the first side of the front plane has a light color. 29.The display of claim 28, wherein the white reflectance of the display isat least about 30%.
 30. The display of claim 1, wherein the rotatingelements are spheres.
 31. The display of claim 30, wherein the sphereshave an average diameter of between about 25 and 150 micrometers. 32.The display of claim 31, wherein the cells have a center-to-centerseparation distance of between about 20 and 200 micrometers.
 33. Thedisplay of claim 1, wherein the plurality of electrodes on the backplaneare substantially coplanar.
 34. The display of claim 1, wherein thechannel has a cross sectional area of at least about 5% of thecross-sectional area of the cell.
 35. The display of claim 1, whereinthe channel does not span the entire height of the channel wall.
 36. Thedisplay of claim 1, in which the cells are arranged in a hexagonal closepacked pattern.
 37. The display of claim 1, in which the cells arearranged in a square close packed pattern.
 38. The display of claim 1,in which the matrix comprises a region in which the cells are arrangedin a square close packed pattern and a region in which the cells arearranged in a hexagonal close packed pattern.
 39. A method of assemblinga front plane of a rotating element display, the method comprising:providing a matrix comprising (i) a support surface; (ii) a plurality ofcells defined by walls on the support surface in the matrix and (iii) atleast one channel through at least some of the walls and connecting atleast some of the cells with one another; disposing a plurality ofoptically anisotropic elements in said plurality of cells; providing atleast one electrode on a side of the matrix opposite said supportsurface; and drawing a dielectric fluid into said cells trough the atleast one channel, whereby in the front plane produced by said methodsaid elements can rotate from a first orientation to a secondorientation within their respective cells when an electric field isapplied to the cells.
 40. The method of claim 39, further comprisingattaching the front plane to a back plane comprising plurality ofelectrodes distributed in two dimensions on the backplane, wherein eachelectrode allows independent control of a discrete region of thedisplay.
 41. The method of claim 39, wherein the optically isotropicelements have at least two colors, and wherein when said elements are inthe first orientation one color is presented to the first side of thefront plane and when said elements are in the second orientation adifferent color is presented to the first side of the front plane. 42.The method of claim 41, wherein the at least two colors are black andwhite.
 43. The method of claim 41, wherein one color of the at least twocolors is lighter than a second color of the at least two colors. 44.The method of claim 43, wherein at least a portion of the matrix visiblethrough the first side of the front plane has a light color.
 45. Themethod of claim 39, wherein the matrix walls, at orientations of theirgreatest height and least width, have an average aspect ratio of atleast about 5:1 (height to width).
 46. The method of claim 39, whereinthe matrix walls, at orientations of their greatest height and leastwidth, have an average aspect ratio of at least about 8:1 (height towidth).
 47. The method of claim 39, wherein the cells are arranged in asquare close packed pattern.
 48. The method of claim 39, wherein thecells are arranged in a hexagonal close packed pattern.
 49. The methodof claim 39, wherein the matrix comprises a first region, in which cellsare arranged in a square close packed pattern and a second region, inwhich cells are arranged in a hexagonal close packed pattern.
 50. Themethod of claim 39, wherein providing at least one electrode on a sideof the matrix opposite said support surface comprises: coating one sideof the electrode with an adhesive; contacting the coated electrode sidewith the matrix; and attaching the electrode to the matrix with theadhesive.
 51. The method of 39, wherein providing at least one electrodeon a side of the matrix opposite said support surface comprises: coatingone side of the matrix with an adhesive; contacting the coated matrixside with the electrode; and attaching the electrode to the matrix withthe adhesive.
 52. The method as in claim 50, wherein the adhesive is aheat-activated adhesive, and wherein attaching the electrode to thematrix comprises applying heat to an assembly comprising the matrix andthe electrode.
 53. A method for loading elements into cells of a matrixof a partially-fabricated electro-optic display, the method comprising:form a suspension of the elements in a fluid, wherein the fluid is inerttowards an exterior material of the elements; contacting the suspensionwith an absorbent transfer member to load the transfer member with aplurality of the elements; contacting the matrix with the loadedtransfer member to transfer the elements into the cells of the matrix;and removing excess elements from the surface of the matrix, wherein theelements have a largest dimension of less than about 1 mm.
 54. Themethod of claim 53, wherein the fluid is a dielectric fluid.
 55. Themethod of claim 53 comprising loading one element per matrix cell. 56.The method of claim 53, wherein the absorbent transfer member is a brushor a sponge.
 57. The method of claim 56, wherein the absorbent transfermember is a brush.
 58. The method of claim 53, wherein the transfermember does not impart a substantial electrostatic charge to theelements during any of the operations of the method.
 59. The method ofclaim 53, wherein excess elements are removed by a second absorbentmember.
 60. The method of claim 53, wherein the elements have a largestdimension of less than about 0.15 mm.
 61. The method of claim 53,wherein: a suspension of the elements in the fluid is a slurry ofoptically anisotropic spheres in a dielectric fluid, said spheres havinga diameter of less than about 0.15 mm; an absorbent transfer member is anon-electrostatic brush; and wherein removing excess elements from thesurface of the matrix comprises removing excess elements with a secondbrush.
 62. A front plane for a rotating element display comprising: afirst side adapted for electrical communication with a backplane; atleast one electrode on a second side of the front plane opposite saidback plane; a light colored matrix comprising a plurality of cellsdefined by walls in the matrix; a plurality of optically anisotropicelements disposed in said plurality of cells wherein the opticallyanisotropic elements have at least two colors, and wherein when saidelements are in the first orientation the light color of the elements ispresented to the first side of the front plane and when said elementsare in the second orientation a darker color is presented to the firstside of the front plane.
 63. The front plane of claim 62, wherein thelight color is white.
 64. The front plane of claim 63, wherein the whitereflectance of the display is at least about 30%.
 65. The front plane ofclaim 64, further comprising a fluid provided in said cells, such thatsaid elements can rotate from a first orientation to a secondorientation within their respective cells when an electric field isapplied to the cells.
 66. The front plane of claim 62, wherein thematrix comprises cells arranged in a hexagonal close pack.
 67. The frontplane of claim 66, wherein the matrix cells are not in fluidcommunication with one another.
 68. The front plane of claim 66, whereineach matrix cell comprises a plurality of the elements.
 69. The frontplane of claim 66, wherein the matrix cell walls are shaped as a moldhosting a plurality of the elements at the wall of each cell.
 70. Afront plane of a rotating element display comprising: a first sideadapted for electrical communication with a backplane; at least oneelectrode on a second side of the front plane opposite said back plane;a matrix comprising a plurality of cells defined by walls in the matrixwherein said walls have an average aspect ratio of at least about 5:1(height to width); a plurality of optically anisotropic elementsdisposed in said plurality of cells.
 71. The front plane of claim 70,wherein the walls have an average aspect ratio of at least about 8:1(height to width).
 72. The front plane of claim 71, wherein the wallsthat define cells of the matrix have a minimum width at regionsseparating adjacent cells of at most about 20% of the diameter or widthof the cells.
 73. The front plane of claim 72, wherein the walls thatdefine cells of the matrix have a minimum width at regions separatingadjacent cells of at most about 10% of the diameter or width of thecells.
 74. The front plane of claim 72, wherein the opticallyanisotropic elements have at least two colors, and wherein when saidelements are in the first orientation a light color is presented to thefirst side of the front plane and when said elements are in the secondorientation a darker color is presented to the first side of the frontplane.
 75. A method of using the rotating element display of claim 1,the method comprising: providing a plurality of signals to at least someof the electrodes of the display; creating a potential differencebetween said electrodes in response to said signals; and rotating atleast some of the optically anisotropic elements from a firstorientation to a second orientation in response to said potentialdifference.
 76. The method of claim 75, wherein when said elements arein the first orientation one color is presented to the first side of thefront plane and when said elements are in the second orientation adifferent color is presented to the first side of the front plane. 77.The method of claim 75, wherein individual signals of the plurality ofsignals address specific electrodes, wherein each electrode allowsindependent control of a discrete region of the display.
 78. The methodof claim 75, wherein the plurality of signals provided to at least someof the electrodes of the display control at least one of a magnitude ofthe potential difference, a polarity of the potential difference and aduration of an electrical pulse.
 79. The method of claim 75, whereinrotating at least some of the optically anisotropic elements creates animage on the rotating element display.
 80. A front plane for anelectro-optic display comprising: a first side adapted for electricalcommunication with a backplane; at least one electrode on a second sideof the front plane opposite said first side; a matrix comprising aplurality of cells facing a viewable surface of the front plane andinterstitial regions outside the cells and also facing the viewablesurface of the front plane, wherein the interstitial regions have afirst color; and a plurality of optical elements disposed in saidplurality of cells wherein the optical elements have said first colorfor display when a pixel of the electro-optic display is a firstelectrical state.
 81. A front plane for an electro-optic displaycomprising: a first side adapted for electrical communication with abackplane; at least one electrode on a second side of the front planeopposite said first side; a matrix comprising a plurality of cellsdefined by walls in the matrix; a plurality of optically andelectrically anisotropic elements disposed in said plurality of cells,wherein said optically anisotropic elements have at least first opaqueexterior region of a first color and a second opaque exterior region ofa second color; and a fluid provided in said cells, such that saidelements can rotate from a first orientation displaying the first colorto a second orientation displaying a second color when an electric fieldis applied to their cells.